Nickel-based superalloy materials are known for use in high temperature, high stress environments such as in the hot combustion gas path of a gas turbine engine. In one application, the nickel-based superalloy known as Alloy 706 (AMS Specification 5701) is used to form the turbine rotor discs of a gas turbine engine. The discs have a generally annular shaped hub portion and an outermost rim portion shaped into a plurality of steeples or dovetails for engaging a respective plurality of turbine blades. Several discs are joined together along an axis of rotation to form a gas turbine rotor.
Turbine discs formed of Alloy 706 have experienced failures during operation. These disks were formed with a two-step heat treatment; i.e. 970° C. solution anneal followed by a 730° C.+620° C. aging treatment (heat treatment B in AMS Specification 5701). This material exhibits a degree of notch sensitivity, i.e., its Larson-Miller Parameter values for a notched bar are lower than those for a smooth specimen at equivalent stress levels, and this is a suspected damage mode for the failed turbine disks. This type of behavior is also known as stress-assisted grain boundary oxidation (SAGBO). To avoid future failures, the failed disks may be replaced with disks formed of a material exhibiting improved notch sensitivity. One example of such a material is Alloy 706 material subjected to a three step heat treatment; i.e. 970° C. anneal followed by a 845° C. stabilizing treatment followed by a 730° C.+620° C. aging treatment (heat treatment A in AMS Specification 5701). Another material that may be used for the replacement disks is Alloy 718 (AMS Specification 5663). However, regardless of the material selected, there is a significant cost associated with the replacement of failed turbine disks.
It is known in the art to repair turbine disks made of low alloy Ni—Cr—Mo—V or Cr—Mo—V steels, such as are used in steam turbine applications. However, repairs have not previously been performed on the stronger nickel-based superalloys that are used in modern gas turbine engines, since fusion welding of such materials in typical disk thicknesses is generally not possible without cracking.